On today's high capacity railroad cars that are used in freight type service, good braking under both empty and loaded weight conditions is a must in order to move goods quickly and smoothly and to satisfy the shippers' demand for "on time" performance.
Single capacity brake equipment produces a brake shoe force that is independent of car loading, thus making it difficult to achieve desirably higher braking ratios for a loaded car without exceeding an empty car braking ratio that would be sufficient to cause the wheel to slide. Sliding wheels are undesirable because of reduced braking retardation.
Special brake equipment is, therefore, necessary to increase the loaded car braking ratio without incurring the consequences of a wheel slide condition when braking an empty car. Such equipment automatically adjusts brake shoe force according to the load conditions of the car. The special equipment falls into two primary categories, dual capacity empty/load braking and multiple capacity or continuously variable braking.
In the dual capacity empty/load equipment, there are just two settings, one for "empty" braking and one for "load" braking. The changeover point between the "empty" and "load" settings in this arrangement is selected at some predetermined car weight. This car weight is usually at 20% of the full load capacity weight. In arbitrarily selecting this changeover point, it will be appreciated that a given car can be generally under-braked by the reduced brake pressure when the car weight is in the upper end of the "empty" weight range, since essentially the same adhesion demand is available at the lower end of the "load" weight range at which the maximum braking force is capable of being supported without sliding the car wheels.
In the variable load type equipment, braking pressure is proportioned according to the actual load, generally throughout the full range of car loading. It will be appreciated, however, that the proportioned brake pressure is selected in accordance with the maximum brake pressure (emergency) capable of being developed from the maximum running pressure normally carried by a train (110 psi). Therefore, when making relatively light service brake applications, or when making a maximum brake application from a relatively low running pressure (70 psi), the proportioned brake pressure may be far less than that capable of being supported by the adhesion demand. Accordingly, less than an optimum brake efficiency will be realized under certain brake conditions with variable load type brake equipment, as well as single capacity equipment, in order to protect against detrimental wheel sliding on an empty car under maximum braking conditions.
In known dual capacity brake systems, such as disclosed in U.S. Pat. No. 3,671,086, this problem is overcome by a proportioning valve arrangement that reduces the braking pressure by a fixed ratio during empty car braking. In order to be compatible with the quick service limiting valve requirements for freight brake control valves, which assures that 8 to 12 psi brake cylinder pressure will be developed from even the lightest service application, the aforementioned system withholds the empty/load sensing function until a predetermined minimum brake cylinder pressure has developed, generally 12 psi.
However, since as equalizing volume is required in such proportioning type dual capacity systems to maintain proper control valve operation, an inadvertent loss of braking pressure can occur when the empty/load changeover valve sensing function comes into play, due to existing brake cylinder pressure being able to momentarily backflow into the equalizing volume. This occurs when a light service reduction is made, just sufficient to operate the load sensing valve on empty cars, the brake cylinder pressure can build up to 12 psi. and then be reduced back to the limiting valve pressure setting on each car, which can be as light as 8 psi. This problem is overcome by the empty/load valve device disclosed in U.S. Pat. No. 5,005,915.
Only two compact air valves and a small volume reservoir are needed to convert single capacity freight car brake equipment to a simplified empty and load operation. A load sensor valve is used to measure the relationship of the car body to the top of the truck side frame to determine if the car is empty or loaded and a load proportional valve which controls the air flow to the brake cylinder when an empty car condition exists. Generally the brake pressure for an empty car is 50 or 60% of that applied for a full car for each amount of brake pipe reduction. The small volume reservoir provides the necessary volume to maintain satisfactory pressure relationships for various brake pipe reductions on the empty car.
An entirely different type of problem is encountered in the use of empty/load braking devices in some railroad cars. In particular, a problem arises in some cars manufactured in foreign countries where there is insufficient displacement between an empty and a load condition for the sensor arm of the standard empty/load sensor valve to travel its full displacement. Thus, since the distance for sensor arm travel is restricted it makes the standard empty/load equipment inoperable.